Silicone-based polymers are one of the most important polymer classes, due to their wide applications within automotive, health care, and electronic device packaging industries. Silicone based materials have advantages such as high transparency in the UV-visible region, controlled refractive index, stable thermo-mechanical properties, and tunable hardness from soft gels to hard resins. Compared with pure silicone polymer, enhanced mechanical, thermal, electric, dielectric and optical properties can be realized with silicone based nanocomposites by adding functional inorganic nanoparticles. However, the incompatibility between inorganic nanoparticles and an organic matrix normally leads to large agglomerates of the inorganic nanoparticles within the polymer matrix. Such agglomeration leads to a loss in optical transparency and severely limits the use of these materials in optical applications. This result is especially true for silicone nanocomposites because the surface energy difference between inorganic nanoparticles and silicone is very large, creating an even stronger tendency for inorganic fillers to form agglomerates within a silicone-based polymer matrix, when compared to most other organic polymers.
Conventional methods for improving compatibility and dispersion of nanoparticles within such polymer matrices adopted by other researchers include physical sonication and surface ligand engineering. However, these techniques only result in visibly transparent silicone nanocomposites, when relatively thin films are made using a spin-coating method. The high transparency was mainly attributed to its very small thickness and kinetically trapped well dispersed nanoparticles during the fast spin-coating process. Preparation of thick transparent silicone nanocomposites with relatively high nanoparticle loading is very challenging.
Surface ligand engineering of spherical nanoparticles (NPs) to tailor nanoparticle dispersion is one of the grand challenges limiting our ability to harness the potential of nanofilled polymers. The basic principle behind surface ligand engineering is the need to shield the surface of the NP to reduce van der Waals (vdW) core-core attraction, while optimizing the wettability or entanglement of the matrix with the surface ligands. Neither small molecule modification nor monodisperse grafted polymer brushes, in general, achieve stable NP dispersion in bulk polymer matrices. For example, coupling agents such as silanes or surface ligands with carboxylic, amine or other reactive end groups only provide limited success in improving the NP dispersion within a solvent or in monomers, due to inadequate steric hindrance. For monodisperse (mono-modal) grafted polymer brushes, minimizing enthalpic interaction requires high surface coverage or a high value of σ√{square root over (N)} with σ being the brush graft density, and N the number of mers. At matrix molecular weights of interest for commercial use, the entropic penalty is often too high for matrix entanglement, as predicted by the scaling criterion σ√{square root over (N)}>(N/P)2 for a polymer matrix with a degree of polymerization P. Instead, “autophobic dewetting” occurs due to the entropic attraction. In order to reduce the entropic penalty and achieve matrix/brush penetration, either the graft density is decreased at a risk of an insufficient screening effect, or a smaller molecular weight matrix is adopted, which is of little technological importance. This balance between the core/core attraction and entropic repulsion has been modeled by Pryamtisyn et al., and qualitative agreement with experimental data has been shown.
As such, a need exists for improved transparent silicone nanocomposites with relatively high nanoparticle loading.